Summary and Analysis

Organic Chemistry: Structure of Alkanes

Introduction to Organic Molecules

Nomenclature and Isomerism, page 2

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Isomerism in Branched Alkanes

In n-alkanes, no carbon is bonded to more than two other
carbons, giving rise to a linear chain. When a carbon is bonded to
more than two other carbons, a branch is formed. The smallest branched
alkane is isobutane. Notice that isobutane has the same
molecular formula,
C4H10
, as n-butane but has a different
structural formula. Two different molecules which have the same
molecular formula are isomers. Isomers which differ in the
connectivity of bonds are constitutional isomers, or structural
isomers. Isobutane is a constitutional isomer of n-butane. The
prefix "iso" indicates that branches off of the central carbon are
equivalent.

Figure %: The constitutional isomers butane and isobutane.

n-butane and isobutane are the only constitutional isomers of
C4H10
. Pentane,
C5H12
, has three while hexane,
C6H14
, has five.

Figure %: Constitutional isomers of pentane and hexane.

Nomenclature of Alkanes

Isobutane, neopentane, etc. are trivial names that arise from
common usage. As you can see, the number of isomers increases rapidly
for larger alkanes. It would be impractical to give trivial names to
every isomer. What is needed is a systematic, easy-to-use method of
naming that works for even the most complex of molecules. Such a name
should unambiguously identify the structural formula of the named
molecule. This system is IUPAC nomenclature, devised by the
International Union of Pure and Applied Chemists.

The IUPAC system considers molecules in terms of a parent
hydrocarbon chain with substituents attached to it. The parent is
the longest continuous carbon chain in the compound, and the base name
of the compound is the alkane that corresponds to the parent chain.
Then, consecutively number the carbons of the parent chain in such a
way that the substituents are attached to carbons with lower
numbers. The name of the compound is the parent alkane prefixed by
its substituents and their position numberings.

Figure %: Correct and incorrect IUPAC names for isopentane.

The -CH3 group is called a methyl group. In general, alkyl
substituents are derived from the corresponding alkanes by replacing
the -ane suffix with -yl. These substituents are used so
frequently that they are given abbreviated names. For instance, methyl
groups can be abbreviated as -Me. In some instances, the exact nature
of the substituent is unimportant. In such cases the notation
-R can be used to denote a radical group, a general substituent that
can be any organic component.

Figure %: Common alkyl substituents and their abbreviated names.

Sometimes there is more than one possible choice of parent chains. In
such cases, choose the parent chain whose substituents are least
substituted.